1. Field of the Invention
The present invention relates to a nonvolatile semiconductor storage device. For example, it relates to a flash memory that has a memory cell capable of storing multivalued data.
2. Background Art
In general, a nonvolatile semiconductor storage device, such as a NOR flash memory, performs the word write operation in which the memory cells are written in units of one word (16 bits) and the page write operation in which the memory cells are written in units of a plurality of words to reduce the write time.
Recently, flash memories having greater capacities have been manufactured at lower costs, and the multivalued technique for storing a plurality of bits of data in a single memory cell has also progressed (see Japanese Patent Laid-Open No. 2005-108303, for example).
If the NOR flash memory has multivalued memory cells, the number of words that can be written in one page write operation is 512, which is significantly greater than 8, 16 or 32, which are the numbers of words that can be written according to conventional techniques. This is due to the increase in write speed and adoption of the error-correcting code (ECC) to improve the reliability, for example (see Japanese Patent Laid-Open No. 2001-210082, for example).